1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet having improved magnetic and surface film properties.
2. Description of the Related Art
The grain-oriented electrical steel sheet is used as a core material of transformers, generation, and other electrical equipment, and therefore, is required to have not only good magnetization and watt-loss properties but also a good surface film.
A grain-oriented electrical steel sheet is obtained by utilizing a secondary recrystallization phenomenon in which crystal grains having a {110} plane parallel to the rolled surface and a &lt;001&gt; axis parallel to the rolling direction are developed. The secondary recrystallization occurs in a final texture annealing step. To ensure a complete manifestation of the secondary recrystallization, an inhibitor such as AlN, MnS, MnSe, or other fine precipitates must be present in steel to suppress growth of primary-recrystallized grains until the steel is heated to a temperature region in which the secondary recrystallization manifests during the final texture annealing. To ensure a complete dissolution of an inhibitor forming element such as Al, Mn, S, Se, and N in steel, the electrical steel slab is heated to a high temperature of 1350.degree. to 1400.degree. C.
The inhibitor forming elements completely dissolved in the electrical steel slab are precipitated in the form of a fine particle of AlN, MnS, MnSe, etc. by annealing a hot-rolled strip or by intermediate annealing prior to a final stage of cold rolling.
In this process, an electrical steel slab is heated to a high temperature as mentioned above, which causes a formation of a great amount of molten scale or slag, and in turn, requires frequent mending of a heating furnace, raises maintenance cost, reduces the availability factor of equipment, and raises fuel cost per unit weight of product.
To eliminate these drawbacks, studies have been carried out to develop a process for producing a grain-oriented electrical steel sheet in which the heating of an electrical steel slab is conducted at a lower temperature.
Japanese Unexamined Patent Publication (Kokai) No. 52-24116, for example, discloses a process in which an electrical steel slab contains Zr, Ti, B, Nb, Ta, V, Cr, Mo and other nitride forming elements, besides Al, so that the slab heating can be carried out at a temperature of from 1100.degree. to 1260.degree. C.
Japanese Unexamined Patent Publication (Kokai) No. 59-190324 discloses a process in which an electrical steel slab contains carbon in an amount as low as 0.01% or less and selectively contains S, Se, Al, and B and the surface of a steel is repeatedly heated or pulse-annealed in a primary recrystallization annealing after cold rolling, so that the slab heating can be carried out at a temperature of 1300.degree. C. or lower.
Japanese Examined Patent Publication (Kokoku) No. 61-60896 discloses a process in which an electrical steel slab contains 0.08 to 0.45% manganese and 0.007% or less sulfur, i.e., has a small value of the product [Mn][S], and also contains Al, P, and N, so that the slab heating can be carried out at a temperature of 1280.degree. C. or lower.
Based on the process of Japanese Examined Patent Publication (Kokoku) No. 61-60896, the present inventors and others proposed an improved process in Japanese Patent Application No. 1-91956, i.e., a process for producing a grain-oriented electrical steel sheet having improved magnetic and surface film properties, in which a final cold-rolled strip is nitrided while it is travelling, and thereby, an inhibitor is introduced into the strip.
The above-mentioned conventional processes, however, has a drawback in that a glass film of a final product sheet sometimes contains a defect called "pepper-and-salt" or "bare spots".